1. Field of the Invention
The present invention relates to a process for producing an optical waveguide, in particular, a flexible polymer optical waveguide, at low costs.
2. Description of the Related Art
As the process for producing a polymer optical waveguide, the following processes are proposed: (1) a process of impregnating a film with a monomer, exposing a core portion selectively to light to change the refraction index thereof, and sticking a film thereto (selective polymerization); (2) a process of applying a core layer and a clad layer, and forming a clad portion by reactive ion etching (RIE); (3) a process employing photolithography to perform exposure and development (direct exposure) using an UV-curable resin obtained by adding a photosensitive material to a polymeric material; (4) a process employing injection molding; (5) a process of applying a core layer and a clad layer, and exposing a core portion to light to change the refraction index of the core portion (photo bleaching), or the like processes.
However, the selective polymerization process (1) has a problem of sticking of the film, and the processes (2) and (3) involve increased costs since photolithography is used. The process (4) has a problem of poor precision of the resultant core diameter, and the process (5) has a problem of an insufficient refraction index difference. Currently, practically used processes having superior performance are only the processes (2) and (3). Any of the processes (1) to (5) cannot be applied to the formation of a polymer optical waveguide on a large-area and flexible plastic substrate.
As the process for producing a polymer optical waveguide, there is known a process of filling a pattern substrate (clad) having patterned grooves to form capillaries, a polymer precursor material for a core, curing the precursor material to form a core layer, and then adhering a flat substrate (clad) onto the core layer. However, this process has a problem in that the polymer precursor material is thinly supplied to not only the capillarity groove but also the entire space between the pattern substrate and the flat substrate and then cured to form a thin layer having the same composition as the core layer, so that light leaks out through this thin layer.
As one of the methods of solving this problem, Davit Heard proposed a method of fixing and sticking a pattern substrate having patterned grooves to form capillaries to a flat substrate using a clamping jig, sealing the contact portion between the pattern substrate and the flat substrate with a resin, and then reducing the internal pressure to fill the capillaries with a monomer (diallyl isophthalate) solution, thereby producing a polymer optical waveguide (Japanese Patent gazette No. 3151364).
However, this method has a problem in that due to its complicated procedure, in which the monomer solution penetrates the portions other than the core portion without using a clamping jig for adhering the plate, thereby failing to produce a precise waveguide structure and an additional problem in that the volume shrinkage occurs when the monomer is polymerized (cured) to form a polymer, thereby altering the shape of the core. Further, when capillaries are removed, the polymer produced from the monomer solution partly adheres to the capillarity, to thereby destroy the core shape.
Recently, George M. Whitesides et al. in Harvard University have proposed, as a method for forming a nanostructure, a method called capillarity micromold as a soft lithographic process. This method uses photolithography to form a master substrate, utilizing adhesiveness of polydimethylsiloxane (PDMS) and easily-peelability thereof to transfer the nanostructure of the master substrate onto a mold made of PDMS, pouring liquid polymer into this mold by capillarity, and curing the polymer. A detailed review thereof is described in SCIENTIFIC AMERICAN September 2001 (Nikkei Science, 2001, December).
Kim Enoch et al. of George M. Whitesides' group in Harvard University obtained a patent on the capillarity micromold method (U.S. Pat. No. 6,355,198).
However, even if the production process disclosed in this patent is applied to the production of a polymer optical waveguide, it takes much time to form its core portion since the sectional area of the core portion of the polymer optical waveguide is small, thus making the process unsuitable for mass production. This process also has a drawback that when a monomer solution is polymerized to form a polymer, a volume change occurs to alter the shape of the core, and consequently, the transmission loss (guided wave loss) becomes large.
The present inventors proposed a method of forming a polymer optical waveguide, which achieves precise core shapes, and remarkably suppressed transmission loss and insertion loss by utilizing capillarity phenomenon. This method, however, requires a prolonged time to fill a curable resin when the core portion (waveguide) is long and hence impairs productivity.
To solve the above problems, Sugiyama et al proposed a method utilizing a plurality of filling inlets (e.g., Japanese Patent Application Laid Open (JP-A) No. 2002-90565). This method has an advantage in that a branched ring-shape waveguide can be formed as it is, however it has a disadvantage in that plural branched filling inlets must be removed precisely, which inevitably leads to increased costs as well as guided wave loss.